This invention relates to improvement in solenoid pilot operated valve using a spool/piston assembly which is operated by fluid pressure in a pilot chamber to switch an input pressurized fluid from one to the other output port.
FIG. 1 of the accompanying drawings shows a known solenoid pilot operated valve, wherein the communication between an input port 2, output ports 3a and 3b and drain ports 4a and 4b is controlled by a spool valve 5 which is slidably received in the valve casings in the usual manner. The spool valve 5 with pistons 8a and 8b is driven by the fluid pressure which is led to pilot valves 7a and 7b through axial passages 6a and 6b in the casings 1, respectively. The pilot valves 7a and 7b include plungers 11a and 11b, respectively, which are axially movable for connecting the pilot chambers 9a and 9b, which are provided on the outer sides of pistons 8a and 8b, either with axial passages 6a and 6b or with drain ports 10a and 10b, respectively.
One of the drawbacks of the change-over valve construction shown in FIG. 1 is that the axial passages leading to the respective pilot chambers through the spool and piston casings are complicated and require additional efforts for sealing those passages fluid-tight, preventing the use of a compact valve construction.
FIG. 2 shows another known solenoid pilot operated valve construction, in which the problem of complication is solved to a certain degree by providing axial fluid passages 14 in the body of the spool valve 13. The fluid pressure is thus supplied through the spool 13 to pilot valves 12a and 12b with plungers 17a and 17b which respectively cooperate with pilot valve seats 16a and 16b on the outer end faces of pistons 15a and 15b which are fixedly threaded on the opposite ends of the spool 13.
The valve construction of FIG. 2 also has an inherent drawback in that the pistons 15a and 15b which are fixedly mounted on the opposite ends of the spool 13 are moved together with the spool 13 through a stroke length several times greater than the length which is necessary for the operation of the plungers (cf. the stroke length of plungers 11a and 11b in the valve construction of FIG. 1) As a result, when the piston is in the remotest position from the plunger, it is necessary to move the plunger 17a or 17b through a greater distance in order to close the pilot valve seat 16a or 16b. The increase in the stroke length of the plungers 17a and 17b also makes difficult the designing of the springs 18a and 18b which urge the plungers toward the pilot valve seats 16a and 16b, respectively, and requires pilot valves 12a and 12b of greater power. The valve as a whole presents a construction which is still remote from being "simple and compact".
It is an object of the present invention to provide a solenoid pilot operated valve which will eliminate the above-mentioned drawbacks, more specifically, a solenoid pilot operated valve of simple and compact construction in which the piston at the end of the spool is movable independently of the spool to return to its initial position upon draining the pilot chamber, thereby facilitating the pilot valve seat closing operation by the plunger.
It is another object of the present invention to provide a pilot operated change-over valve which is simple in construction and easy to fabricate.